The present invention relates to pervious diaphragms based on inorganic fibers such as asbestos intended for cells for the electrolysis of aqueous solutions of alkali metal halides such as sodium chloride or potassium chloride. More particularly it relates to diaphragms of stabilized thickness, that is to say diaphragms whose thickness remains substantially constant during their whole working life, directly deposited on foraminate cathodes. The invention also relates to a method for the manufacture of such diaphragms and to electrolytic cells equipped with such diaphragms.
To manufacture an asbestos diaphragm directly on the foraminate cathode of an electrolytic cell, it is known, from U.S. Pat. No. 1,865,152 in the name of K E STUART, of June, 28, 1932, to disperse asbestos fibers in an aqueous solution, to immerse the cathode in the suspension of asbestos thereby obtained, then to suck the suspension through the foraminate cathode. During suction of the suspension through the foraminate cathode, the asbestos fibers are retained on the cathode where they progressively build-up the diaphragm.
In the known method, the aqueous solution may be a solution of sodium chloride or potassium chloride or an alkaline solution obtained from a diaphragm cell in which a sodium chloride or potassium chloride brine is being electrolyzed.
The advantage of this known method resides in its simplicity and in the capability of applying asbestos diaphragms with precision onto cathodes of complex cross-section. It is generally used in the case of cells with interleaved vertical electrodes, of the type described in Belgian Pat. Nos. 780,912 and 806,280 in the name of the present applicant, applied for resepctively on 20.3.72 and 19.10.73.
The diaphragms obtained by this known method have however the disadvantage of suffering changes in thickness, often large changes, during the course of electrolysis. Thus, during the first weeks of use, these diaphragms generally begin to swell, with the detrimental result a considerable increase of the ohmic resistance in the diaphragm. Furthermore, this swelling of the diaphragm interferes with the release of the chlorine produced at the anodes. In order to avoid accelerated deterioration of the diaphragm by erosion owing to turbulent release of chlorine, it is necessary to construct the cells so that the distance between the anodes and the cathodes is large and generally greater than 10 mm, even as much as 15 mm. All other things being equal, this entails the two-fold disadvantage of increasing the space occupied by the cells and reducing the energy yield of the electrolysis.
To avoid these disadvantages of diaphragms obtained by this known method, there has been proposed, in Belgian Pat. No. 809,822 of Jan. 16, 1974, of DIAMOND SHAMROCK CORPORATION, a method wherein an aqueous suspension of asbestos fibers and fibers or particles of a thermoplastic polymer is formed, the suspension is sucked through the foraminate cathode to deposit on it a diaphragm formed of a substantially homogeneous mixture of the asbestos fibres and the polymer, and the diaphragm is heated at high temperature, for example above 300.degree. C., to melt the polymer and allow it to bind together the asbestos fibers.
Although this known method allows the dimensional stability of asbestos diaphragms to be improved, it still has the disadvantage that it is expensive because it involves the use of polymers that are difficult to manufacture. Moreover, the method is critical and risky to carry out. It is specially difficult to ensure a homogeneous dispersion of the polymer among the asbestos fibers. Also, the fusion of the polymer necessitates heating to very high temperatures, which not only considerably burdens the cost of manufacturing the diaphragms but often causes distortion of the cathode.
To improve the dimensional stability of asbestos diaphragms it has also been proposed, in German Pat. No. 1,696,259 of Mar. 18, 1967, of SIEMENS AG, to treat the asbestos with a solution of alkali metal hydroxide and afterwards to heat the diaphragm formed on the cathode, between 300.degree. and 700.degree. C. This known method allows a reduction in the tendency of asbestos diaphragms to swell during use in electrolytic cells. However, it has the same disadvantage of requiring a thermal treatment which is expensive and is likely to damage the cathode.
In order to improve the firmness and the mechanical properties of diaphragms not directly formed on the cathode but made from sheets of asbestos fibers, it has been proposed, in U.S. Pat. No. 3,694,281 of Apr. 9, 1969, in the name of J A LEDUC, to impregnate the asbestos sheets with a liquid medium containing a polymer, then to heat the impregnated sheets at high temperature, so as to melt the polymer.
This known method has the disadvantage of requiring a long and expensive thermal treatment. It has the further and important disadvantage of affecting the permeability and the hydrophilic nature of the diaphragms, the molten polymer having a tendency to block the pores formed between the fibers of asbestos.